In current commercial TV transmission standards, the limited bandwidth of the transmitted chrominance (or chrominance difference) signals causes the received images to have perceptibly blurred color transition edges. This is especially evident if the received image contains geometrical patterns, e.g., test-color bars, and results in the loss of detail detectable in complex multicolored fine patterns.
In order to improve the quality of the received images, it is necessary to provide the receiver end with circuits capable of restoring, as far as possible, the frequency components in the chrominance signals which have been filtered away by the requirements of the reduced transmission bandwidth. In this way, the temporal duration of the chrominance transition edges, and thus the spatial extent of the chrominance transitions on the TV screen, can be reduced, and the edge definition improved. Circuits of this type are called "Color Transient Improvement" ("CTI") or Chrominance Transition Enhancement circuits.
An important constraint on chrominance transition enhancement circuits is the need to ensure that the center of the chrominance transition is unaffected by the enhancement process, so that the center of the chrominance transition after the enhancement process is still aligned with the center of the associated transition in the luminance signal. Also, it is necessary to leave gradual transitions in time unaltered; preserve, and possibly enhance, fine patterns; prevent the introduction in the image of additional distortions; and ensure that the existing noise components are not accentuated.
Most of the methods proposed over the past decade to enhance chrominance transitions make use of analog waveform shaping techniques, and do not satisfy most of the above-mentioned requirements.